I have yet to perform some more in depth tests - logging out and then in again, or restarting - to see if GB remains the same, or has corrected itself.
[Editor's note: I tested this to the same extent as the hint author, and it seems to be true. Put a copy of a huge file in the trash, make sure the disk or volume the item is on is selected in the Finder, and empty the trash. The space available will decrease!]

